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Chapter 7
Operating
Systems
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OBJECTIVES
After reading this chapter, the reader should
be able to:
Define the purpose and functions of an operating system.
Understand the components of an operating system.
Understand the concept of virtual memory.
Understand the concept of deadlock and starvation.
List some of the characteristics of popular operating systems
such as Windows 2000, UNIX, and Linux.
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Figure 7-1
Computer System
Hardware is the physical devices but Software is the collection of
Programs that allows the hardware to do its job.
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7.1
DEFINITION
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Note:
An operating system is an interface
between the hardware of a computer
and
the user (program or human) that
facilitates the execution of the other
programs and the access to
hardware and software resources.
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7.2
EVOLUTION
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-Batch Systems. The programmer does not have any control
or interactions during the execution of his Jobs (programs).
-Time-Sharing Systems efficiently use computer resources
because it allows several programs to share resources. Each
program is allowed to use the resource only if it is available
and also for a portion of time. Therefore, OS do scheduling:
Allocating the resources to different programs and deciding
which program should use which resource when.
- Personal Systems serve single user and needs single-user
operating system (e.g DOS).
- Parallel Systems with multiple CPU on the same machine
serve one or more programs in parallel but with more speed.
-Distributed Systems allows programs to run partially on
one computer and partially on another computer on networks
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7.3
COMPONENTS
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Figure 7-2
Components of an operating system
Today Operating systems are very complex. In order to manage resources
An OS has at least 4 manager where each one manages one resource
But can cooperate with others.
Users can communicate with OS either directly or through user Interface.
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Figure 7-3
Memory Manager
Memory
is a limited resource that should
Monoprogramming
be managed efficiently. In order to run
Programs, memory manager allocates
enough memory for programs and data.
-There are two categories of memory
management: mono- and multi-programming
1- Monoprogramming:
-It allows one and only one Program to
be in memory! It loads a program into
memory, run it, and replace it with
the next program when it is finished.
-Program size can NOT exceed the size of
Memory!
- Very inefficient usage of memory and CPU
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Figure 7-4
Memory Manager
2- Multirogramming:
Multiprogramming
-More than one program can be in memory
at the same time and are executed
Concurrently.
The CPU switches between Programs.
-When I/O operations are executed by a
running program, CPU is not idle but can
schedule another program to run.
-It uses memory and CPU efficiently.
-It uses different techniques (swapping,…)
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Figure 7-5
Categories of multiprogramming
-Nonswapping means that the programs remains in memory for the
duration of execution.
-Swapping means that the program can be swapped between memory
and disk one or more times during execution
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Figure 7-6
Partitioning
Memory is divided into variable length sections (partitions). Each section
holds one program NOT part of it. The CPU switches between programs.
Partitioning improves CPU efficiency but it suffers from :
1- if partition size is set too small, some programs can't be loaded into
memory and if it is set too large, holes (unused locations) will be created.
2- Holes may be created after programs are replaced by new ones
3- Removing holes and create new partitions creates extra overhead.
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Figure 7-7
Paging
Paging improves the efficiency of partitioning. In paging memory is
divided into equally sized sections called frames and programs are divided
into equally sized sections called pages. Page size usually equal frame
size. Pages of entire program needs enough frames(not necessary contiguou
in memory to run. To improve paging, several variants techniques do exist:
1-Demand paging: The entire program does not need to be in memory to ru
but pages can be loaded into
memory one by one.
2- Demand segmentation: The
program is divided into segments
(modules) and loaded into memory,
executed, and replaced by another
module of the program or another
3-Demand Paging & segmentation
memory is divided into frames &
a module can be divided into pages
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Figure 7-8
Virtual memory
Demand paging and
Demand segmentation means
that part of the program is in
memory and part is on disk.
Example: a memory of
10 MB (physical memory)
can execute 10 programs,
each of 3 MB for total of
30 MB (virtual memory).
At any moment 10MB
Of the 10 program are in
memory and 20MB on disk.
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Process Manager
A second function of an OS is process management. To
understand process management we need to define the
following:
-Program is a nonnative set of instructions written by
programmer and stored on disk (or tape). It may or may not
become a job.
-A program becomes a JOB from the moment it is selected
for execution until it has finished running and becomes a
Program again!
-Process is a program in execution. It is a program that has
started but not finished. It is a job that is residing in memory
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Figure 7-9
State diagram with the boundaries
between a program, a job, and a process
A program becomes a Job when
OS selects it and brought it into
Hold state.
When there is enough memory
to load the program totally or
partially, the job becomes a
process and moves to the
Ready state. The process
remains in this state
until CPU is given to it and
it moves to running state. It
remains there until it needs I/O
or its allocated slot of time is
finished or process terminate.
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Figure 7-10
Job scheduler
To move a job or process from one state to another, the process manager
uses 2 schedulers; job and process schedulers.
-JOB SCHEDULER moves a job from hold state (create a process)
to ready state or from running state to the terminate state.
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Figure 7-11
Process scheduler
It moves a process from one state to another. It moves a process from
running to waiting if it waits for some events to happen. It moves a
process from Running to Ready if time allotment has expired. It moves a
process from waiting to ready if the event has occurred. It moves a proces
from ready to Running if CPU is available.
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Figure 7-12
Queues for process management
To handle multiple processes and jobs competing for resources, process
Manager uses QUEUES (waiting lists). Associated with each process or
Job is a job or process control block (JCB or PCB)that stores information
about each Job or process. OS stores JCB or PCB in the queues and leaves
the jobs or processes themselves on disk. OS can use different policies
for selecting the next job or process from a queue. Examples are
FIFO or Shortest Length First or one with highest priority, and so on.
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Process Synchronization
The whole idea behind process management is to synchronize
different processes with different resources. When resources
can be used by more than one user (process), 2 situation arise
namely deadlock and starvation. Those situations should be
avoided.
-Deadlock can occur if OS does not put restrictions on using
resources. To avoid deadlock 4 conditions should happen:
1-no more than one process can hold a resource(mutual exclusion)
2-no process can hold a resource that it can't use until other
resources are becoming available (resource holding)
3-OS can temporarily reallocate a resource (preemption)
4- no circular waiting (all processes and resources form a loop)
-Starvation is the opposite of deadlock and it can occur if
OS puts too many restrictions on using resources.
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Figure 7-13
Deadlock
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Figure 7-14
Deadlock on a bridge
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Note:
Deadlock occurs when the
operating
system does not put resource
restrictions
on processes.
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Figure 7-15.a
Starvation
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Figure 7-15.b
Starvation
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Figure 7-15.c
Starvation
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Figure 7-16
Dining philosophers
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Device Manager
It is responsible for access to input/output devices and also
for the efficient use of devices. In general device manager:
-monitors constantly the status of every I/O device (free,
in use, have problem,..)
-Maintains a queue for each I/O device or one or more queues
for similar I/O devices.
-Controls the different polices for accessing I/O devices.
It may use FIFO for one device and SJF for another
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File Manager
In general the responsibilities of file manager are:
-control access to files (read/write/execute).
- supervise the creation, deletion, modification of files
-Control the naming of files
-Supervise the storage of files (how and where they are stored)
-Archiving and backups of files
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7.4
POPULAR
OPERATING
SYSTEMS
Windows, UNIX and Linux are popular OS. Each one has
Its own user interface which is a program that accepts requests
From users (processes) and interprets them for the rest of OS.
In UNIX, LINUX , windows it is the SHELL or command inte
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